From the Manual
Remember the silly Viking missions that took soil samples to detect life on Mars and missed the big rock shaped aliens? Well, now we're at war with those oversized pet rocks. They've managed to terrorize space with a strange assortment of ships, creatures and weird asteroids. Hop in your cruiser and show them who's boss in this solar system.
The object (besides having fun) is to follow the fighters back to their base and destroy it. If you successfully vaporize the base, you will be refueled, supplied with more ammunition and returned to open space at the next higher level of difficulty. When you reach the next level you must again pursue the enemy fighters back to their base.